Wave form analyzing method for establishing fourier coefficients



MOON ROOM Dec. 22, 1964 \K.,H. HAAsE WAVE FORM ANALYZING METHOD FOR ESTABLISHING FOURIER COEFFICIENTS Original Filed Sept. 18; 1959 2 Sheets-Sheet 1 Dec. 22, 1964 K. H. HAAsE 3,162,808

WAVE FORM ANALYZING METHOD FOR ESTABLISHING FOURIER COEFFICIENTS Original Filed Sept. 18, 1959 2 Sheets-Sheet 2 Pile/a JIM/76W JIM/raw 77 J3 IN1/EN TOR. Afa/er /x A44/wf BYLUW MMI?. 711 "w- United States Patent C Original application Sept. 18, 1959, Ser. No. 840,981. Divided and this application Nov. 18, 1960, Ser. No.

6 Claims. (Cl. 324-77) (Granted under Iitle 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This is a division of original application, Serial No. 840,981, filed September 18, 1959, now U.S. Patent 3,009,106, by the same applicant and relates to analysis of electrical signals in accordance with their harmonic content and more particularly to a novel method for obtaining any desired number of Fourier coefficients of the periodic functions which constitute a complex periodic wave.

It is well known that any periodic wave which is single valued and continuous may be represented by the sum of an infinite number of sine waves of different amplitudes and frequencies. This is represented by the following equation and is known as a Fourier series:

f(t)=A/2 -lil am cos 21rmt+ bm sin 21rmt (l) where 1 1F21 fwd: (2)

@nf-2g fo) cos 2mm (s) bml=2L1 fo) sin 2mm (4) It is therefore only necessary to obtain an adequate number of values for the coefficients AO/z, am and bm (known as Fourier coefficients) to provide an accurate expression for any nonsinusoidal periodic wave.

It is a common problem in the fields of engineering, applied physics and mathematics to obtain these coefficients and specifically the subject invention may be used in investigations of speech sounds, X-ray examinations of the crystal structure of materials, the field of propagation, and seismic exploration and color matching through the medium of spectro-photometric curves.

Known methods of determining the equation of a complex wave include experimental observation, the use of oscillograms, the use of a comb lter to filter out the harmonic frequencies and the use of synthesizers which generate component waves and vary their phase, frequency and amplitude to simulate the wave to be analyzed.

These analyzing methods are limited to a finite number of coefficients or by the total bandwidths of filters and synthesizers.

The present invention avoids the aforementioned limi- ICC tations of known Fourier analyzing methods by teaching a novel technique of deriving, instead of the elementary natural coefficients defined by Fourier, periodic coefficients, each of which is multiplied by real number factors which are functions of the number of harmonic orders desired and the number of segments the wave has been divided into.

Accordingly, it is necessary to obtain only a certain limited number of periodic coeicients and to multiply these periodic coefficients by applicable real number factors to deliver any arbitrary number of natural Fourier coefficients.

It is therefore the principal object of this invention to teach a novel method of determining any desired number (without limit) of Fourier coefficients as the product of a periodic cofiicient and applicable real number factors.

The novel features which I believe to be characteristic of my invention are set forth with particulanity in the appended claims. The invention itself, however, together with further objects and advantages thereof can best be understood by reference to the following description taken in connection With the accompanying drawings, in which:

FIGURE 1 illustrates a typical periodic non-sinusoidal wave and its approximate equivalent consisting of a polygonial sequence of straight lines; and

FIGURE 2 is a block diagram illustrating one possible means for performing the subject method.

Referring now to said drawings, the period of a function (t) (dotted) which is in sufficient correspondence with the polygonial sequence of straight lines f(l) (solid lines) is shown to be divided into an integral number of segments in the t (time) direction. If a periodic function f(t) is approximated by a polygonial trace of straight lines, when the Fourier coefficients am and b,m are substantially the same for f(t) and for its approximation. I have found that if the corners of the polygon trace are equidistant in the direction of the l-axis, so that the interval of the period is divided into n equal sections, then the coefficients can be written as products efficients. These parameters are further defined by the following equations and table:

tao

b*m= 2 y, sin (10) m CmI m Cm2 m Cm? 'm Cm Table of Factors Cm2 for parameter N =9.

I have discovered, and it is fundamental to my invention, that periodic coeicients a*m and b*m are, under certain conditions, common parameters of intercorrelated groups of periodic functions which constitute the wave to be analyzed. More specifically I have found that in a periodic wave which has been segregated into n timeW-ise equidistant segments, as illustrated in FIGURE l, the periodic coefficients for intercorrelated groups of periodic functions are identical at points i, vr-, 1r-land 21r-. This is true also of periodic coeicients at points 21', 1r-2, 1r+2 and 21r-2 and for other intercorrelated groups up to N -i. Therefore, to solve for any number of Fourier coefficients it is only necessary to obtain n sample y Values of the Wave to be analyzed, group said sample y values as indicated and multiply the groups by the number factors Kn and Cm.

To facilitate grouping of values, the following equations have been formulated in which the notations involving group summations have a systematic background in the choice of capital letters and indices which makes their distinction easy. Coefficients, iand related functions thereof, associated with said intercorrelated groups of periodic functions are designated by an asterisk, said asterisk being a general symbol intended to differentiate the periodic functions comprehended by my invention from the socalled natural Fourier functions. N is an integer defined as Capital letters H and F are summations of 4 distinct sample values, y1, iy2n4, iylm, and |yn4. Capital H is correlated to even indexed coetiicients a*2n, capital F to odd indexed coeicients a*2,u+1. According to indices 2i or 2i-1, H and F make a choice between group summations beginning with even or odd numbered samples y2, or 3,214. For computation of b*n1 coeicients, these capitals are marked by a bar. Capital letter G is used in summations involving even indices 2i in H and F as well as in the trigonometric function. Capital letter U is used where 2i-1 appears instead of 2i. The indices of G and U may be, corresponding to m, 2n or Zn-I-l. Again barred capitals and are used for computation formulas for btm coefficients.

Using Hi defined in (12) we get Introducing the sums defined in (l1) (14) into Equs. 9 and l0 We define now two subgroups of sums, namely Sui sums Gm where only even indexed sums F21, H21, F24,

H21 are included and Sa sums Um where only odd indexed sums F214, H214,

F214 and H224 are included. Furthermore, we discriminate between even indices me=2,u and oda ndices m0=2p-1. So, we dene, if N odd=2y-1 (17a) 'y-l il Hei-1 If N even=27 we deline It is obvious, from a review of the unique concepts and equations developed herein, that each step of the subject method invention may be performed manually. It is equally obvious that the individual steps, either separately or collectively, may be readily adapted to automatic computing devices. Typical of such automatic computing devices is the Wave Form Analyzing Apparatus illustrated by the lblock diagram of FIG. 2, which Form Analyzing Apparatus is the subject of my co-pending patent application, Serial No. 840,981, wherein it is described in detail. Basically, the subject analyzing apparatus comprises three stages; a period recognizer and storage stage, an operational circuit stage, and an attenuator stage. The period recognizer and storage stage wherein each of the 36 samples on FIG. 1 (y0=0, y1, y2 Y35, y36=0) is stored up to the end of the computation process comprises sweep oscillator means 4, period recognizer 5, sampler switches 6, 10 and 13, position collector switches 7 and 8, adder means 9, and potentiometer means 11. The operational circuit stage comprehends all the circuitry required to compute the periodic coefficients and includes collector switches 14, 15, 16, 17, 19, 20, 21, 22, 33 and 34, program switch array 18, converters 23 and 24, adders 25 through 28, and sampler switches 29 through 32 arranged as shown. The operations involved are, sampling and collecting of values derived from the sample valves y0 yas, additions and polarity conversions, and, finally, multiplication of sample or derived values by real values 1. The attenuator stage provides automatic means for deriving natural coefficients from the periodic ones and includes attenuator means 39, sampler switches 37 and 38, phase meter 35, and root-mean-square meter 36 arranged as shown.

There has thus been disclosed novel concepts and equations which, when carried out either manually or by means of mechanical apparatus, provide an improved method of performing a Fourier analysis in accordance with the objects set forth herein.

What is claimed is:

l. A method of performing a Fourier analysis comprising the steps of establishing one complete period of the non-sinusoidal function to be analyzed, measuring amplitude values of said period at a plurality of intervals equally spaced in time, grouping said amplitude values according to their common harmonic orders, deriving iirst and secon-d real number factors, and multiplying said groups by said first and second real number factors, said first real number factor being a function of the number of Fourier coeicients desired, and said second real number factor being -a function of the number of amplitude values taken.

2. The method of determining Fourier coefficients of a nonsinusoidal periodic function whichconsists of taking a number of sampleamplitude values at intervals equally spaced in time of the nonsinusoidal function to be analyzed, collecting said sample values according to common harmonic groups of the sinusoidal components which constitute said nonsinusoidal function, deriving first and second real number factors, and multiplying said common harmonic groups by real number factors which are a function of the number of Fourier coefficients desired and the number of equal sample values taken.

3. A method of obtaining any desired number of Fourier coeicients of a nonsinusoidal periodic Wave cornprising the steps of establishing the period of the wave to be analyzed, taking a number of amplitude values equally spaced in time of one period of said wave, collecting said sample values into groups according to their common harmonic orders, deriving first and second real number factors, and multiplying said groups by real number factors. said real number factors being dependent upon the number of Fourier coetiicients desired and the number of sample values taken.

4. A method of establishing any desired number of Fourier coeiiicients consistingy of establishing the period of the wave to be analyzed, dividing said wave into an integral number of amplitude values equally spaced in time, grouping said amplitude values according to an integral number of equal sections of the component sinusoidal wave periods which constitute the wave to be analyzed to form groups of periodic functions, intercorrelating said groups of periodic functions in accordance with their predetermined mathematical relationships, deriving real number factors which are functions of the number of harmonic orders desired and the number of segments the wave to be analyzed has been divided into, and multiplying each of said intercorrelated groups by their corresponding real number factors to obtain the desired number of natural Fourier coeicients.

5. A method of performing a Fourier analysis of a nonsymmetrical period wave comprising the steps of establishing the period of the Wave to be analyzed, dividing said wave into an integral number of amplitude values equally spaced in time, grouping said amplitude values according to their common harmonic orders, deriving a first real number factor which is a function of the number of amplitude samples taken and the number of Fourier coefficients desired, multiplying said groups by said first real number factor, summing such products with corresponding amplitude values to form periodic Fourier coeicients, deriving a second real number factor which is a function of the number of amplitude values taken, and multiplying said periodic Fourier coefficients by said second real number factor to obtain the natural Fourier coefficients.

6. The method of determining the Fourier coecients of a nonsinusoidal periodic function f(t) for any desired harmonic order m comprising the steps of establishing the period of the wave to be analyzed; dividing said period into n equal parts i; grouping amplitude values y taken at tra, wirk, and 21r-tk1 according to the sum and difference of the sums and differences of the means and extremes of said Values y for 11/4-1 values of index integer k; multiplying such grouped values by real number coeicient sin -lrm/n m sin 1r/n grouping the products with the value of y taken at 1r,

according to even and odd Values of index integer k; and multiplying said groups by the coeicient where n is an integral number of samples, z' is a particular part, tkt, vritki and 21r-tki are particular points along the period of the wave to be analyzed where sample values y are taken, and m, n and z' cover the range of the number of natural Fourier coeflicients desired.

References Cited by the Examiner UNITED STATES PATENTS WALTER L. CARLSON, Primary Examiner.

SAMUEL BERNSTEIN, Examiner. 

1. A METHOD OF PERFORMING A FOURIER ANALYSIS COMPRISING THE STEPS OF ESTABLISHING ONE COMPLETE PERIOD OF THE NON-SINUSOIDAL FUNCTION TO BE ANALYZED, MEASURING AMPLITUDE VALUES OF SAID PERIOD AT A PLURALITY OF INTERVALS EQUALLY SPACED IN TIME, GROUPING SAID AMPLITUDE VALUES ACCORDING TO THEIR COMMON HARMONIC ORDERS, DERIVING FIRST AND SECOND REAL NUMBER FACTORS, AND MULTIPLYING SAID GROUPS BY SAID FIRST AND SECOND REAL NUMBER FACTORS, SAID FIRST REAL 